This invention relates generally to equipment used to purify natural gas and more particularly concerns dehydrators for removing water from natural gas.
Natural gas is presently dried by use of trayed towers, perhaps 1.5 to 9 feet in diameter and 20 or more feet tall. The gas is passed from a spreader at the bottom of the tower to a small outlet at the top of the tower while liquid desiccant, such as triethylene glycol spurted into the top of the tower against approximately 1,000 psi flows to bottom of the tower as each tray overflows to the next in zig-zag fashion. Each tray interposes an approximately one inch thick lamina of desiccant in the path of the gas. Since there are typically 5 to 9 trays in a tower, every gas molecule is expected to pass through 5 to 9 inches of desiccant. The wet desiccant at the bottom of the tower is then heated to boil off the water and the pure desiccant is pumped to the top of the tower. For example, if triethylene glycol is used, the wet glycol is heated to approximately 375.degree. F. to boil off the water.
There are serious disadvantages in using the trayed tower equipment. A first disadvantage is that the diameter of the dried gas outlet at the top of the tower, typically is 1/6 the diameter of the trays. Since the gas tries to flow along the most direct path to the outlet, the gas flow pattern is generally conical. Thus, while the trays and, therefor, the glycol laminae are of equal diameter, the unused area of each sequentially higher lamina concentrically increases until, at the top of the tower, very little of the glycol is put to use. A second disadvantage related to the first is that, as tower diameter is increased, the inefficiency of the unit is also increased because the unused area is a function of the square of the diameter. Thus, while an 18" diameter tower might require 3 gallons of glycol to absorb 1 pound of water, a 9' diameter tower might require 4 gallons of glycol to absorb 1 pound of water. This leads us to a third disadvantage which is that, even for small diameter towers, more glycol must be heated in the boil-off process than was effectively used in the drying process and, for larger diameter towers, energy is wasted at an increasing rate. A fourth disadvantage is that the trayed tower system blows an initial gas molecule through a small hole in the bubble cap associated with the tray. Subsequent gas molecules tend to follow the path cleared by the initial molecule and, therefor, may not be fully contacted by the desiccant. A fifth disadvantage is that, since the tower configuration provides 5 to 9 one inch laminae of desiccant, the residence time of the gas in the desiccant is quite limited, further reducing the efficiency of the equipment. A sixth disadvantage of trayed tower dehydrators is that, although the gas is relatively volatile, the gas flow pattern is so linear that there is too little interaction of the molecules to promote useful contact with desiccant between laminae. A seventh disadvantage of trayed tower dehydrators is that they cannot be used in floating production system operation or FPSO such as a tanker retrofitted with production equipment on the deck. The angle and alignment of the trays must be maintained to assure the uniform flow of desiccant across each tray and overflow between trays. The natural motion of a ship or tanker floating on a body of water prohibits this necessary uniform flow and overflow. Most of the future offshore drilling and production will be in water one to two miles or more deep. This is impossible with fixed leg platforms and, therefore, all such drilling and production will be done using FPSO systems.
It is, therefor, an object of this invention to provide a dehydrator for and method of dehydrating natural gas in which the flow path of the gas is sinusoidal along a horizontal axis into and out of the liquid desiccant. Another object of this invention is to provide a dehydrator for and method of dehydrating natural gas in which the flow path of the gas is sinusoidal between zeniths and nadirs radially distant from the longitudinal axis of the dehydrator. Yet another object of this invention is to provide a dehydrator for and method of dehydrating natural gas which uses the liquid desiccant available for the drying process and the energy required in the boil-off process more efficiently than in presently known dehydrators. Still another object of this invention is to provide a dehydrator for and method of dehydrating natural gas in which a greater percentage of the gas molecules come in full contact with the liquid desiccant while they are in the desiccant pool than in presently known dehydrators. A further object of this invention is to provide a dehydrator for and a method of dehydrating natural gas which afford greater residence time of the gas in the liquid desiccant than presently known dehydrators. Another object of this invention is to provide a dehydrator for and method of dehydrating natural gas which take advantage of the volatile nature of the gas to cause the gas to come into useful contact with liquid desiccant even when the gas is not in the desiccant pool. It is also an object of this invention to provide a dehydrator for and method of dehydrating natural gas which is useful in floater applications.